ARTICLE pubs.acs.org/crystal
Surface Crystallization of Cordierite from Glass Studied by High-Temperature X-ray Diffraction and Electron Backscatter Diffraction (EBSD) Wolfgang Wisniewski,* Carlos Andre Baptista, Matthias M€uller, G€unter V€olksch, and Christian R€ussel Otto-Schott-Institut, Jena University, Fraunhoferstrasse 6, 07743 Jena, Germany ABSTRACT: A glass with the composition of stoichiometric cordierite (2Al2O3 3 2MgO 3 5SiO2) shows surface crystallization during thermal annealing. High temperature X-ray diffraction (HT-XRD) was performed on a compact sample. The surface crystallization is further studied using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The formation of an oriented layer at the immediate surface is not observed. This is in disagreement with previous reports in the literature. Topographical effects during crystallization and EBSD pattern acquisition are analyzed by atomic force microscopy.
1. INTRODUCTION The crystallization of a glass forming melt can be considered as a phase transformation from a nonequilibrium amorphous state into a thermodynamically more stable crystalline state (in many cases the equilibrium state) at the respective temperature. The crystallization of the glass may either start in the bulk or from the surface. Some glass systems nucleate homogeneously in the bulk (e.g., Li2O 3 SiO2, Na2O 3 SiO2, CaO 3 Al2O3 3 2SiO2), while heterogeneous or surface crystallization is predominantly observed in other systems (e.g., PbO 3 SiO 2 , Na 2 O 3 2SiO 2 , CaO 3 MgO 3 2SiO2).1 A wide range of parameters may affect nucleation and crystallization; a number of parameters have been reviewed for various glass compositions, e.g. in ref 2. The first general description concerning the thermodynamics and kinetics of glass crystallization, including aspects of surface crystallization, was given in 1933 by Tammann;3 later, overviews were given in numerous books (see, e.g., refs 4 and 5). The crystallization of cordierite and its polymorphs6 8 has been the topic of numerous publications and was summarized extensively in some reviews.9 12 Annealing a glass with the composition 2Al2O3 3 2MgO 3 5 SiO2, that is, with the same composition as cordierite, solely results in surface crystallization.1,2 One explanation for the exclusive surface crystallization is the inhibition of bulk crystallization by elastic strains originating from the different specific densities of the glass and the formed crystals, see, for example, ref 5, p 302. Mainly two phases are observed in thermally annealed samples: a low temperature phase with β-quartz structure, often denoted as μ-cordierite or high-quartz solid solution,13 and a high temperature phase known as indialite or α-cordierite depending on annealing temperature. The kinetics of crystal growth has been described r 2011 American Chemical Society
in refs 14 16; however, the mechanism is still not fully understood. The crystallization in this system is accompanied by a loss of mass (mainly water)16 and leads to decreasing densities from the glass (2.62 g/cm3) to μ-cordierite (2.59 g/cm3) and finally to indialite (2.49 g/cm3).17 Slightly different values for the densities are given in ref 18. Decreasing densities are quite unusual for a glass crystallization process, however, not unique. In the literature, in situ scanning electron microscopy (SEM) at high temperatures was reported, and a high growth rate of the μ-cordierite crystals at the surface after an initial induction period is described.19,20 In surface crystallized cordierite, a thin layer of glass first detected via optical microscopy was reported to cover the crystals which led to a new view on the theoretical aspects of surface nucleation.14 Crystallization studies performed using high temperature X-ray diffraction (HT-XRD) reported that μ-cordierite is initially formed at 900 °C and transforms to indialite at 950 °C.13 The temperatures for the formation of the respective phases are reported to be lower if powdered glass is annealed (grains
